Capacitive sense arrays may be used to replace mechanical buttons, knobs and other similar mechanical user interface controls. The use of capacitive sense elements permits elimination of complicated mechanical switches and buttons to provide reliable operation under harsh conditions. In addition, capacitive sense elements are widely used in modern customer applications, providing new user interface options in existing products. Capacitive sense elements can be arranged in the form of a capacitive sense array for a touch-sensing surface. When a conductive object, such as a finger, comes in contact or close proximity with the touch-sensing surface, the capacitance of one or more capacitive touch sense elements changes. The capacitance changes of the capacitive touch sense elements can be measured by an electrical circuit. The electrical circuit converts the measured capacitances of the capacitive sense elements into digital values.
Transparent touch screens that utilize capacitive sense arrays are ubiquitous in today's industrial and consumer markets. They can be found on cellular phones, GPS devices, cameras, computer screens, MP3 players, digital tablets, and the like. In contemporary cellular phones and smart phones, touch screen area is of significant concern to manufacturers given the small amount of space available for user interaction. As such, manufacturers seek a touch screen made of layers of transparent materials that are as thin as possible. However, conventional thin layer designs exhibit considerable sensitivity to noise.
FIG. 1 illustrates a conventional pattern design of a capacitance sense array panel 100. The capacitance sense array panel 100 comprising an N×M sense element matrix which includes transmit (“Tx”) electrodes 102 and receive (“Rx”) electrodes 104. The transmit and receive electrodes 102, 104 in the N×M sense element matrix are arranged so that each of the transmit electrodes intersects each of the receive electrodes. Thus, each transmit electrode 102 is capacitively coupled with each of the receive electrode 104. For example, transmit electrode 102 is capacitively coupled with receive electrode 104 at the point where transmit electrode 102 and receive electrode 104 intersect. The intersection of the transmit electrode 102 and the receive electrode 104 is called a sense element. Although the intersection of the transmit electrodes 102 and the receive electrodes 104 are called sense elements, the electrodes themselves are referred to herein as sense elements since the electrodes can be disposed in a sense array that is used for mutual capacitance sensing, as well as in a sense array that is used for self capacitance sensing. It is noted that the embodiment disclosed in FIG. 1, the orientation of the axes of Tx electrodes may be switched with the Rx electrodes as would be appreciated by one of ordinary skill in the art having the benefit of this disclosure. It is also noted that the Tx electrodes 102 form straight parallel lines 106 along an axis 110 that is perpendicular to an axis 112 of the Rx electrodes 104 that also form straight lines 108.
Because of the capacitive coupling between the transmit and receive electrodes, a Tx signal (not shown) applied to each transmit electrode induces a current at each of the receive electrodes. For instance, when a Tx signal is applied to transmit electrode 102, the Tx signal induces an Rx signal (not shown) on the receive electrode 104. When a conductive object, such as a finger, approaches the N×M matrix, the object will modulate the signal by changing the mutual capacitance at the intersections of the Tx and Rx electrodes. Since a finger would normally activate about three to five neighboring intersections, a signal profile can be readily obtained. Finger location can therefore be determined by the distribution of this profile using a centroid algorithm.
FIG. 2 illustrates a conventional routing of liquid crystal display (LCD) lines 210 relative to the arrangement of electrodes and their axes of orientation. The (LCD) lines 210 generally run straight, parallel, and below parallel sets of Rx sense elements 220 connected to each other and to the same electrode (i.e., the column 230 or the RX electrode 230). LCDs may be configured to energize a column 230 of sense elements 220 at a 60-120 Hz rate. In such circumstances, a voltage that energizes the column 230 capacitively couples charge into sense elements 220. While a sensing circuit is making a measurement, the LCD control signal on the LCD column 210 couples the charge onto column 230 as noise. The more coupling that exists between the straight LCD lines 210 and the straight parallel column 230 of the Rx sense elements 220, the higher the noise amplitude induced into the Rx sense elements 220 due to the LCD column 210.